The present invention relates to nucleic acid constructs and cells, and further to methods utilizing same for modifying the electrophysiological function of excitable tissues. More particularly, embodiments of the present invention relate to the use of cells having gap junctions and ion channels or transporters for modifying the electrophysiological function of excitable tissues.
The biological cell membrane, the interface between the cell and its environment, is a complex biochemical entity one of whose major involvement is the directed transport of specific substances. A related major involvement of the cell membrane is the maintenance of chemical gradients, particularly electrochemical gradients, across this interface. These gradients are of great functional significance (e.g., in the production of action potentials in nerve and muscle cells).
Ion channels are macromolecular protein pores, which span the cell membrane lipid bilayer. While approximately 30% of the energy expended in cells goes to maintain the ionic gradient across the cell membrane, it is the ion channel that dissipates this stored energy, much as a switch releases the electrical energy of a battery.
Ion channels are efficient compared to enzymes; small conformational changes gate a single channel between “closed” and “open” states, allowing up to 107 ions to flow in one second, amounting to approximately 10−12 Amperes of highly selected ions flow during the channel opening. Since they are efficient, the number of ion channels per cell is relatively low; a few thousand channels of a given subtype/cell are usually sufficient to perform their task while orders of magnitude higher numbers of receptors or enzymes are required to carry out their tasks.
Ion channels are usually classified by the type of ion they selectively pass (sodium, potassium, calcium, or chloride) although some are indiscriminate. Different ion channels are activated (or gated) by either extracellular ligands, transmembrane voltage, or intracellular second messengers.
Ion Channel Conductance
Conductance quantifies the ease with which ions flow through a material and is expressed in units of charge/sec/volt. Single channel conductance, g, as distinguished from the membrane conductance (G) of the entire population of channels, is defined as the ratio of single channel current amplitude (i) to the electromotive force, or voltage (V):g=i/V 
The direction of ion movement through channels is governed by electrical and chemical concentration gradients. Entropy dictates that ions will flow passively through ion channels down a chemical gradient. Electrically charged ions will also move in an electrical field, just as ions in solution flow to one of the poles of a battery connected to the solution. The point at which the chemical driving force is just balanced by the electrical driving force is called the Nernst equilibrium (or reversal) potential. Above or below this point, a particular ion species will flow in the direction of the dominant force. The net electrical flow across a cell membrane is predictable given the concentrations of ions, the number, conductances, and selectivities of the channels, and their gating properties.
The modern method of deciphering ion channel function is by using patch clamp technology. In the patch clamp technique, a small polished electrode is pressed against the plasma membrane. For unknown reasons, the affinity between glass and cellular membrane is incredibly high; very few ions leak through this tight seal. In essence, the electrode isolates and captures all ions flowing through the 1-3 square microns of the cell membrane defined by the circular border of the glass pipette. The result is that the ionic current passing through a single ion channel can be collected and measured. The current through the attached patch (cell-attached), a detached patch (inside-out or outside-out), or the whole cell can be measured.
Ion Channel Building Blocks
Since ion channel function is easily measured in real time, most ion channels were cloned using the South African clawed toad (Xenopus laevis) oocyte. These oocytes are large enough to inject with exogenous mRNA and are capable of synthesizing the resulting foreign proteins. In expression cloning, in vitro transcripts (mRNA) from a cDNA library derived from a source of tissue/cell known to be rich in a particular current are injected into individual oocytes. The proteins encoded by this library are allowed several days to be translated and processed before the oocyte currents are measured by voltage clamp techniques. The cDNA library (with ˜1 million unique clones) is serially subdivided until injected messenger RNA from a single cDNA clone is isolated that confers novel ion channel activity. Moreover, mutant cDNA clones with engineered alterations in the protein's primary structure can be expressed and the ion channel properties studied in order to determine regions of the protein critical for channel activation, inactivation, ion permeation, or drug interaction.
The building blocks for most channel proteins are individual polypeptide subunits or domains of subunits each containing six hydrophobic transmembrane regions labeled S1 through S6. The Na+ and Ca2+ channel pores are single (a) subunits in which 4 repeats of the six transmembrane spanning domain surround the pore. Voltage-gated K+ channels (Kv; nomenclature refers to K channel, voltage-dependent) are encoded by a tetramer of separate six-transmembrane spanning motifs. Coassembly of the linked domains form the central pore and confer the basic gating and permeation properties characteristic of the channel type. The peptide chain (H5 or P loop) juxtaposed between the membrane spanning segments S5 and S6 project into and line the water-filled channel pore. Mutations in this region alter the channel's permeation properties. S4 is probably the major channel voltage sensor since it contains a cluster of positively charged amino acids (lysines and arginines). Voltage-dependent channel inactivation is mediated by a tethered amino terminal blocking particle (called the ball and chain mechanism) which swings in to occlude the permeation pathway (inactivation). Amino acids in the S6 transmembrane segment participate in another inactivation pathway named C-type inactivation.
The most recently discovered family of channel proteins are the inward rectifier K+-selective channels (Kir; K channel, inward rectifier). These channels determine the resting membrane potential in most cells because they are open at rest. Kir channel topography is similar to the Kv channel class but the subunits lack the S1 to S4 segments present in Kv channels. The two transmembrane spanning domain surrounding the conserved H5 pore domain is deceptively simple; heteromultimeric channel formation, direct G protein gating, and interactions with other proteins by some Kir subtypes considerably increases the complex behavior of this channel class.
Ion Transporters:
Yet another class of molecules which participate in ion transport across cellular membranes are the ion transporters. Ion transporters are integral membrane proteins capable of pumping one ion out of the cell while pumping another ion into the cell.
In, for example, Na/K ion transporters, the Na+, K+ pump activity is the result of an integral membrane protein called the Na+, K+-ATPase. The Na+, K+-ATPase consists of a “catalytic” α-subunit of about 100,000 daltons and a glycoprotein β-subunit of about 50,000 daltons. When operating near its capacity for ion transport, the Na+, K+-ATPase transport three sodium ions out of the cell and transport two potassium ions into the cell for each ATP hydrolyzed. The cyclic phosphorylation and dephosphorylation of the protein causes it to alternate between two conformations, E1 and E2. In E1 the ion-binding sites of the protein have high affinity for Na+ and face the cytoplasm. In the E2 conformation the ion-binding sites favor the binding of K+ and face the extracellular fluid.
Examples of other ion transporters include the Na/Ca exchange system which participates in regulation of intracellular Ca+; the Na/H exchange system which function in concert with a Cl/HCO3 exchange system to regulate intracellular pH; and the Na—K—Cl exchange system which contributes to smooth muscle function and which is regulated by a number of vasoactive agents.
Excitable Tissues
Myocardium: Myocardial contraction depends on the opening and closing of a complex series of ion channels in myocardial cell membranes.
The most prominent of these channels are the K+ Ca++ and Na+ ion channels.
The number of K+ ions is greater inside a resting myocardial cell than outside. But the number of Na+ ions is greater outside. When a myocardial cell beats, sodium channels open allowing a rapid, transient in-rush of Na+ ions, then close within about two one-thousandth's (2/1000) of a second. This depolarizes the membrane with the positive ions moving in. Then there is then a slower, but prolonged (½ second), release of potassium to the outside of the cell which repolarizes the cell membrane.
Although myocardial contraction is more complex and involves other ions and channels, the end result of this depolarization-repolarization is that the contractile filaments in the cell engage, and the cell contracts.
Nerve cells: Signal propagation through neuronal cells is also governed by ion influx/outflux through nerve cell membranes. In nerve cells, Na+, Ca++ and K+ channels participate in the generation and propagation of a nerve signal.
Glandular tissue: Secretion of glandular factors, such as hormones is in some cases effected by the excitation of secreting cells or tissues. For example, in the pancreas, T-type calcium channels along with cell-to-cell gap junctions participate in secretion of insulin.
Since ion channels participate in numerous physiological processes, damage to cells and/or channels of excitable tissues can be a cause for numerous disorders.
For example, heart conditions, such as reentrant arrhythmia, are brought about by the damage or death of myocardial cells, which can no longer support normal electrophysiological function. Secretion of factors from glandular tissue, such as insulin is also effected by damage to excitable cells forming this tissue, while nerve cell changes, as for instance in disorders such as epilepsy severely effects nerve signal function.
The present invention provides a novel approach for modifying the electrophysiological property and thus the electrophysiological function of excitable tissues.
This novel approach, which according to one embodiment of the present invention utilizes cellular implants, can be utilized for restoring normal electrophysiological function to damaged tissues such as heart, nerve or glandular tissues.